This invention relates to a process for reducing the sulfite salt content of an aqueous solution. More particularly, it relates to removing the sulfite ion values from an aqueous solution by substantially converting them to bisulfite by reducing the pH of the solution and contacting the solution with a macroreticular organic compound containing carbonyl functional groups which can form an adduct with the bisulfite salts.
Water soluble sulfite and bisulfite salts are known and used in the chemical industry. For example, aqueous sodium sulfite is used to scrub sulfur dioxide from flue gases, and sodium and magnesium bisulfite solutions are used to bleach wood, paper, and the like. Often, byproducts and/or water streams from these uses contain residual sulfite. These residues are undesirable as they represent unused and costly materials and are environmental pollutants.
In the Wellman-Lord process, sulfur dioxide is removed from a flue-gas stream by converting the sulfur dioxide to bisulfite ion. The removal of bisulfite ion or salt from an aqueous solution is taught in U.S. Pat. No. 4,196,175 which is incorporated herein by reference. Ion exchange processes for the removal of bisulfite are also known in the art, for example, U.S. Pat. Nos. 3,879,521 and 3,896,214 which involve the exchange of bisulfite ion for ions such as hydroxide ions on ion exchange resin. The chemisorption of bisulfite salts with a compound containing adduct-forming carbonyl functional groups involves the addition of bisulfite salts to a carbonyl group as an adduct, rather than the exchange of bisulfite ion with another ion.
In the Wellman-Lord process for removing sulfur dioxide from flue gases, sulfur dioxide in the flue gas is reacted with a countercurrent flow of water and sodium sulfite, forming sodium bisulfite. As the process operates, sulfate ion tends to accumulate in the scrubbing solution, which tends to adversely affect the sulfur dioxide scrubbing properties of the Wellman-Lord system. Therefore, it is necessary periodically to purge the system to remove the sulfate ion. By the use of a macroreticular solid with carbonyl groups thereon, as disclosed in U.S. Pat. No. 4,196,175, the bisulfite values in the purge solution can be recovered; however, the sulfite values in the purge stream pass through the macroreticular solid unaffected and are lost. In a Wellman-Lord scrubbing process it is desirable to recover the sulfite as well as the bisulfite ion for economic operation.